A low-cost robust and reliable technical solution is required to address the need for monitoring spa and whirlpool halogenation systems. Owing to the comparatively heavy organic loading of whirlpool tubs, and the potential for foul water to sit in the suction line for extended periods in between usage, there is a need to disinfect the water as it re-circulates. Cartridge-based systems release controlled amounts of sanitizer directly in to the water flow system. Since the cartridge has a finite source of sanitizer, indication is needed to confirm that the cartridge contains a significant amount of sanitizer. Eventually the cartridge will become depleted, necessitating replacement. In the case of the sanitizing agent being a product of slow decomposition of an insoluble solid, it is possible to monitor the level of sanitizing agent by measuring the remaining level of solid matter within the cartridge. There are many techniques for measuring fill level within a container. These include paddle switches, ultrasonic, electromagnetic wave and capacitive measurements. Invariably these measurements assume that the space not filled is occupied by air or vacuum, such that the conductivity is zero and the relative dielectric constant is unity.
Of particular relevance is, for example, U.S. Pat. No. 6,237,412, which describes a capacitative sensing technique for liquid level measurement. This is extended by U.S. Pat. No. 6,269,694 to include powder level measurement. U.S. Pat. No. 4,188,826 describes a resistance measurement for the level of a conductive liquid. Additionally WO95/12704A1 describes sensors for dosing washing machines with detergent by way of measuring the conductivity of the soap solution.
In the field of geophysics the well-known Archie equation describes the relationship between porosity and resistivity in reservoir rocks, and measurement of rock sample resistivity is a standard analytical technique. A method has been developed as described in U.S. Pat. No. 4,158,165 using inductive coupling of radio frequency signals to produce a non-contact sensor which can be used in a bore hole. This teaching has been extended in U.S. Pat. No. 5,811,973 to measure both real and imaginary parts of the sample impedance.
This invention is an improvement over the existing techniques because it uses a well established method for characterizing porous rock samples to determine the fill level of man-made granular, insulating material in an aqueous environment. Additionally, the electrical signal may be extracted from the cartridge by a two-part inductive link. In this way, the simplicity and sensitivity of the direct connection to the porous medium is maintained whilst no conductive elements apart from the measurement electrodes are exposed to the potentially corrosive aqueous environment, leading to long and reliable operation.